Preventing the stack

I would like to preface this by saying that I am away from my school for the next few months. I would ask my instructor or one of the purple belts there, but that option isn't available right now.

As a n00b, I often get thwarted in my attempts to pull of submissions, but I've been having particular trouble with getting stacked on armbars and leg triangles from guard. So far I've been taught to post one of my feet on their hip and use that to prevent them from being able to push forward and stack.

Are there any details that people commonly miss, or does this tend to be a function of not having done armbars and triangles enough?

1) Follow your instructor's advice
...
2) You might not be angled out enough if you keep getting stacked
3) There's some easy sweeps off both armbar and triangle stack attempts, maybe ask your instructor about them

Are the people that are stacking you good or like you?
It makes a difference, there is good stacking and bad stacking.

There's a mix of both. When subject to good stacking, I tend to fail. When subject to bad stacking I tend to be uncomfortable for a while, but eventually pull my sub off.

There's one person in particular who tends to use the stack effectively. He's a bluebelt- wrestler to bjj crossover like myself. Stacking tends to come naturally to wrestler, in my experience so it makes sense that he would be particularly good at it.

Not "more importantly" it really varies on how good you opponent stacks.
Against poorer stacks there are lots of sweeping options. As your opponents get better you'll become limited to rolling through the stack and turning on knees. Better opponents will kill the roll through by leg placement leaving turning on knees and maybe reguarding.

Against good opponents there are a number of sweeps that will work when they're about to finish the pass but that comes down to timing.